Well, my professor was not here today, but I did a little more research on the quake.
USGS maintains a website that shows earthquakes worldwide from the past week, which is a good source of information.
I was a little surprised to see how deep the quake was (about 40 km). Large quakes in Hawaii are generally the result of pieces of the volcanoes shifting around. As the volcanoes build up from the seafloor, the lava cools quickly upon contact with seawater. This leads to fairly steep slopes along the side of the volcano. Once the volcano grows above sea level, the lava is able to flow along the ground for much greater distances, often all the way to the shore, where it is quenched by the ocean. This leads to the very broad, shallow slopes on the volcanoes above sea level. You can imagine what happens when you put millions of tons of basalt spread out broadly on top of a base that is fairly narrow.
Collapses of parts of the shoreline are common in Hawaii, where volcanic benches grow from lava flow with little or no support for them below sea level. I was actually glad to hear that this recent earthquake took place offshore to the NW of the Big Island. One of the greatest hazards on the Big Island is the collapse of the Kilauea Rift Zone. Around 1/4 of the volcano on the southern side of the Island is separating from the rest of it along a line of faults. Eventually, that whole block will separate and slide off into the ocean. Not only would this be accompanied by a very large earthquake, but the tons of rock falling into the ocean would generate an enormous tsunami. Fortunately, this kind of massive event has not happened on Hawaii during recorded history. But the evidence of massive landslides in the past can be found off the coast of all the Hawaiian islands. Ever wonder why the islands get smaller the farther from the Big Island they are? Because they have lost much of their original real estate to these kinds of massive landslides, as well as millions of years of erosion.
I don't want to give people the wrong impression about Hawaii. While there are major geological dangers like these huge landslides, and of course volcanic activity and earthquakes, they are not necessarily imminent threats. Massive collapses of the volcanic edifices happen once every few 100,000 years. That's a much lower risk of it happening in your lifetime than the people living in California have of experiencing "the Big One".
Getting back to the recent earthquake, you wouldn't find a professional seismologist linking the North Korea nuclear test to a quake in Hawaii. The amount of seismic energy that would reach Hawaii from a 4.2 in North Korea is less than the energy generated by a local 1.0 event due to magma movement in the volcanoes. The "why" of the activity in Hawaii will have to do with the growth and decay of the volcanoes. In this particular case, where the quake was so deep within the earth, it would have something to do with the way that the hot spot fueling the volcanoes is interacting with the oceanic crust under Hawaii.